1. Technical Field
The present invention relates to a surface sensing device for use in position determining apparatus such as, for example, a coordinate measuring machine (CMM), a scanning machine, a machine tool or a measuring robot.
2. Related Art
Such machines are used for measuring workpieces, and typically comprise an arm other member which is movable in three directions X, Y and Z relative to a table on which the workpiece is supported. Movements of the movable arm or other member in each of the X, Y and Z directions are measured by transducers on the machine, so that the position of the movable member relative to a datum position can be determined.
The surface sensing device with which the invention is particularly concerned is an analogue or measuring probe, having an elongate stylus with a workpiece-contacting tip. In use, it may be mounted on an articulating head for use in a high speed scanning operation, such as described in our U.S. Pat. No. 5,040,306. The head is mounted on the movable member of the machine, and has motors or actuators capable of orienting the axis of the probe stylus about two orthogonal axes of rotation. Transducers associated with these rotatable axes determine the direction of orientation.
During a scanning operation, the machine and/or the head cause the stylus tip to move over the surface of the workpiece, in accordance with instructions from the machine controller, to gather data about the profile of the workpiece surface. From the signals provided by the transducers of the machine and the head, and from a knowledge of the dimensions of the probe stylus, the positions of points on the surface being scanned can be estimated. However, this would only have the required accuracy if the stylus were made sufficiently rigid, which is impractical.
Our U.S. Pat. No. 6,633,051 (corresponding to International Patent Application No. WO 00/60310) shows such a probe. It includes a relatively flexible, hollow stylus, which bends laterally under the forces of contact between the stylus tip and the workpiece surface, and under inertial forces while accelerating. An optical system is provided which measures the lateral displacement of the stylus tip caused by such bending. This is then combined with the measurements from the transducers of the machine and the head.
The optical system comprises a light beam which passes along the hollow stylus. The beam is then reflected by an optical component at or near the tip, to pass back along the stylus. Lateral displacement of the stylus tip causes a lateral or tilting displacement of the returned beam, which is measured by a position sensitive detector.
The probe shown in U.S. Pat. No. 6,633,051 is capable of measuring only lateral displacements of the stylus tip.